This invention relates to an improved method and apparatus for heating fluids. More particularly, this invention relates to a method and apparatus for heating liquids and agglomerating slurries, preferably above their critical points. The invention also relates to a novel method for controlling the rate of heating of a liquid or slurry.
"Critical point" and the "supercritical region" can be best illustrated by reference to FIG. 1 attached hereto. At ambient temperatures and pressures the vapor and liquid phases of a pure normally liquid substance, such as water, can be distinguished clearly from one another. For example, at a temperature, t, a liquid phase can be produced by applying a pressure, p, which exceeds the vapor-liquid boundary curve, labeled "saturation line." At temperatures equal to and higher than t.sub.c a liquid phase cannot be produced regardless of the pressure applied to the vapor. At the critical point, t=t.sub.c and p=p.sub.c, the vapor and liquid phases and are in fact indistinguishable. When both the temperature and pressure exceed the critical point, the substance is in a "supercritical condition" and is called a "supercritical fluid." At this point only a single phase exists which cannot be defined as either liquid or vapor.
Liquids can be vaporized by passing electricity directly through the liquid, thus using the electrical resistance of the liquid as the heating element of an electric circuit. Conventional home vaporizers operate on this principle. Ott de Lorenzi in Combustion Engineering-A Reference Book In Fuel Burning and Steam Generation (1st ed. 1949 Combustion Engineering Superheater, Inc.) Ch. 17, pp. 17-1 through 17-10, teaches a commercial adaptation of such an apparatus.
The de Lorenzi teaching limits the application of this technique to temperatures and pressures below about 400.degree. F. and about 250 pounds per square inch absolute ("psia"). In addition, de Lorenzi controls the rate of heating by adjusting the water level in the heater. Finally, de Lorenzi teaches that the electrodes in a single unit heater having multiple electrodes must be insulated from each other by an insulating plate to maintain voltage balance.
It is also known that electrically conductive liquids continue to exhibit some conductivity in the supercritical region. See, for example, Marshall, Conductances And Equilibria of Aqueous Electrolytes Over Extreme Ranges Of Temperature And Pressure, 18 Rev. Pure and Appl. Chem. 167 (1968) and the references cited therein. These fluids, however, experience a dramatic drop in conductivity as they pass through the critical point. For example, Quist and Marshall, Electrical Conductances of Aqueous Sodium Chloride Solutions From 0.degree. To 800.degree. And At Pressures To 4000 Bars, 72 J of Phy. Chem. 684 (Feb. 1968), FIG. 4, page 689, show that the specific conductance of a 0.001 molal sodium chloride solution drops from about 70 (ohm.sup.-1 cm.sup.-1).times.10.sup.-5 to less than about 5 (ohms.sup.-1 cm.sup.-1).times.10.sup.-5, a factor of 1400%, as the temperature of the solution is raised from below critical to above critical temperature (specifically from 578.degree. F. (303.degree. C.) to about 768.degree. F. (409.degree. C.)) while the pressure is held at about critical pressure about 3200 psia (217 bars). Thus, the prior art teaching indicates that direct resistance heating cannot practically be used to heat a fluid beyond its critical point.
Heating slurries that agglomerate present additional difficulties. For example, conventional heating techniques for heating liquids and slurries generally involve direct firing or transfering of heat from a hotter material to a cooler material, by a shell-and-tube type heat exchanger. When these conventional indirect techniques are used for heating agglomerating slurries, such as a slurry of coal and water, to supercritical temperature, agglomeration of the coal on the heating surface can result. The agglomerated coal can then clog the system and impede efficient operation and heat transfer. In addition, the heat transfer provided by these conventional techniques is relatively slow. As a result, the equipment often includes lengthy heat exchange tubing, making the apparatus inappropriate for applications with small space and rapid heating requirements.
The present invention is designed to improve upon these and other aspects of the prior art.